PROJECT SUMMARY Intensity is a key factor when considering the dose of physical activity (PA) required to achieve specific health and fitness outcomes. The intensity of aerobic PA can be defined either in terms of absolute intensity or relative intensity. Absolute intensity refers to the energy or work required to perform an activity and does not take into account the exercise capacity of the individual. Relative intensity takes into account or adjusts for a person's exercise capacity. For many obese and/or unfit adults, it may not be possible to achieve moderate- intensity PA as measured on an absolute scale due to their low exercise capacity. However, these adults may be able to perform moderate-intensity PA on a relative scale. There is limited evidence regarding the dose of relative intensity PA necessary to promote and maintain health. Addressing this question has important implications regarding how individuals should monitor their PA intensity, how physicians should prescribe PA, and how exercise interventions should be designed. One reason for the limited amount of evidence on the relationship between relative intensity PA and health is the fact that it is difficult to measure relative intensity. As a result, nearly all large prospective observational studies measure PA intensity on an absolute scale. Wearable monitors (accelerometers) are widely used in both epidemiological and clinical research studies for capturing objective estimates of PA without a large participant burden. These devices convert body movement into derived accelerometer ?counts? and there are several published thresholds or cut-points for classifying PA intensity on an absolute scale based on counts. The Coronary Artery Risk Development in Young Adults (CARDIA) study is an ongoing cohort study of 5,115 African American and White men and women aged 18-30 at baseline in 1985-1986. In 2005-2006, an ancillary CARDIA Fitness Study was performed on 2,760 participants aged 38 to 50 who wore accelerometers during a standardized graded exercise treadmill test and for 1 week of normal activity. Participants were subsequently re-examined in 2010-2011 and 2015-2016 to measure the development of cardiovascular disease risk factors. Using data from the CARDIA study, we will develop and validate statistical methods for objectively measuring relative intensity PA using accelerometry. Our specific aims are: 1) Develop statistical methods for calculating relative-intensity PA accelerometer cut- points and apply these methods to data from the CARDIA exercise treadmill test; 2) Use the CARDIA data to estimate the relationship between relative-intensity PA and the development of cardiometabolic disease as compared to absolute-intensity PA. 3) Investigate the generalizability of our methodology to more translational calibration settings.